The present invention relates generally to ultrasonic transducers, and, more particularly, to an ultrasonic transducer backing assembly constructed using a composite acoustic absorption material and a method for making same.
Ultrasonic transducers have been available for quite some time and are useful for interrogating solids, liquids and gasses. One particular use for ultrasonic transducers has been in the area of medical imaging. Ultrasonic transducers are typically formed of piezoelectric elements. The elements typically are made of material such as lead zirconate titanate (abbreviated as PZT), with a plurality of elements being arranged to form a transducer assembly. Alternatively, ultrasonic transducer elements may be fabricated using semiconductor manufacturing technology in combination with micro-machining technology to fabricate a micro-machined ultrasonic transducer (MUT) on a semiconductor substrate. Such a MUT is described in U.S. Pat. No. 5,619,476 to Haller, et al., the disclosure of which is hereby incorporated into this document by reference.
The MUT""s are formed using known semiconductor manufacturing techniques resulting in a capacitive non-linear ultrasonic transducer that comprises, in essence, a flexible membrane supported around its edges over a substrate, which may be a semiconductor substrate. By applying electrical contact material to the membrane, or a portion of the membrane, and to the substrate, and by applying appropriate voltage signals to the contacts, the MUT may be energized such that an appropriate ultrasonic wave is produced. Similarly, the membrane of the MUT may be used to receive ultrasonic signals by capturing reflected ultrasonic energy and transforming that energy into movement of the membrane, which then generates a receive signal. When imaging the human body, the membrane of the MUT moves freely with the imaging medium, thus eliminating the need for acoustic matching layers.
The transducer assembly (whether PZT or MUT) is then assembled into a housing possibly including control electronics, in the form of electronic circuit boards, the combination of which forms an ultrasonic probe. This ultrasonic probe, which may include acoustic matching layers between the surface of the transducer element or elements and the probe body, may then be used to send and receive ultrasonic signals through body tissue.
Ultrasonic transducers typically operate by delivering acoustic energy to a target to be interrogated and receiving a version of the emitted pulse back as acoustic energy, which has been modified by the target and includes imaging information regarding the target. The received acoustic energy is then converted by the transducer to an electrical signal and processed by electronics to display an image of the interrogated target on a display.
When an electrical pulse excites a transducer element, the transducer emits acoustic energy from both a front surface and a rear surface. The acoustic energy emitted from a front surface is usually directed toward the target that is being interrogated. The acoustic energy emitted from the rear surface, however, may cause difficulties with the signal that is received from the target. This interference happens when acoustic energy directed from the rear surface of the transducer interferes with acoustic energy received from the target that is under interrogation. The acoustical energy that is directed from the rear of the transducer may create acoustic oscillations, thus causing interference with the acoustic energy received from the target.
Furthermore, a potential drawback of ultrasonic transducers is that some of the acoustic energy generated during a transmit pulse, and some of the acoustic energy received during a receive pulse, is transferred into the substrate on which the transducer is formed. This acoustic energy transferred to the substrate may be in the form of xe2x80x9cLamb wavesxe2x80x9d, or other acoustic waves, that may interfere with the operation of the transducer. Lamb waves are waves of acoustic energy that travel through a thin plate of material parallel to its surfaces, and in this instance may be said to travel parallel to a surface of the substrate. Furthermore, a portion of this acoustic energy may be coupled back into the transducer""s active area, thus causing significant interference with the operation of the transducer.
To minimize the detrimental effects of the aforementioned acoustic interference, transducer assemblies typically include backing material. The backing material performs a number of functions. First, the backing material may provide a mechanical support for the transducer or the transducer array, as transducers are typically formed in arrays including a number of individual transducer elements. The backing material may also provide for attenuation, or absorption, of the acoustic energy emitted from the rear surface of the transducer, thus minimizing the above-described acoustical interference. The backing material is typically constructed of a material that includes electrical contact material.
Typically, the electrical contact material is formed in, or added to, the backing material to provide an electrical connection through which an excitation pulse may be communicated from control circuitry to the transducer element and through which a receive pulse may be communicated form the transducer element to the control circuitry.
A drawback of this backing material is that the electrical contacts formed therethrough, or included therein, are difficult to precisely locate within the backing material such that they provide proper connection between the transducer elements and the control circuitry without the electrical contacts coming in contact with each other. This is a significant drawback when lead spacing uses fine pitch (where electrical contacts are spaced on the order of 250 microns or less) technology. Another drawback of this backing material is that the thermal coefficient of expansion of the backing material is frequently different than that of the electrical conductors associated therewith. Furthermore, the TCE of the backing material is also frequently different than that of the control circuitry and of the transducer elements that the backing material is located between. Unfortunately, this undesirable condition leads to failures in the electrical connections between the backing material and the control circuitry and leads to failures in the electrical connections between the backing material and the transducer elements.
Therefore, it would be desirable to have a backing material that can effectively reduce or eliminate the acoustic energy projected from the rear of a transducer. It would be desirable for this backing material to have a thermal coefficient of expansion that closely matches that of the electrical contact material used to connect a transducer to control circuitry and that reduces fabrication difficulties.
The invention provides a backing for an ultrasonic transducer, comprising a first planar substrate including a first surface. The first planar substrate is configured to acoustically couple to the ultrasonic transducer. Electrical contact material applied to the first surface of the first planar substrate is configured to electrically couple to the ultrasonic transducer.
The present invention may also be conceptualized as a method for making a backing for an ultrasonic transducer, comprising the following steps: forming a first planar substrate to include a first surface and configured to acoustically couple to the ultrasonic transducer; and applying an electrical contact material to the first surface. The electrical contact material is configured to electrically couple to the ultrasonic transducer.